Ionization-type smoke detectors are known and are widely recognized as useful in detecting airborne smoke indicative of a fire. Some of the known ionization-type sensors usually include a chamber having a portion open to the ambient atmosphere (active chamber) and a portion which is relatively sealed (reference chamber).
The chambers are often separated, at least in part, by a conductive sensing, electrode. Two exterior electrodes are spaced from the sensing element by the active and the reference chambers respectively.
A radioactive source(s) can be provided to inject alpha particles into the two chambers. As is known in the prior art, an electrical potential is usually applied between the two exterior electrodes. In this configuration and as biased, the two chambers behave as high impedance resistors with a small current flow between the two exterior electrodes. Known sources suitable for use in ion chambers include Americium241.
In response to changing ambient smoke conditions, which permeate into the active chamber, the voltage measurable at the sensing electrode will vary. This varying voltage is indicative of a smoke level and when compared to a pre-established alarm threshold can be used to detect the presence of an alarm condition.
As is known, ionization-type smoke detectors are conventionally placed on the ceilings of the regions being monitored. Known ionization-type sensors extend from the respective ceilings an amount which is determined at least in part, by the height of the ionization chamber.
Architectural preference has evolved in a direction which calls for low profile detectors. Such low profile detectors tend to be less easily noticed than detectors which extend from the ceiling a greater distance. Hence, shorter ionization chambers are preferable.
The miniaturization of ionization chambers inside smoke detectors generally causes a reduction of operating current, with the consequent reduction of stability and reliability of the detector. This result follows from inspection of the Bragg curve for alpha particles in air. Such a curve was published by authors Holloway & Livingston in 1938, in the paper, "Range & Specific Ionization of Alpha Particles." If the radiated alpha particles impinge upon an ionization chamber wall before creating their full allotment of ion pairs, then potential operating current in the ionization chamber is lost.
FIG. 1 illustrates a Bragg curve based on the above noted publication but extended by approximation beyond a range of 2.8 cm. The extended portion of FIG. 1, beyond 2.8 cm, is consistent with a corresponding curve published in "Study of Ionization Curves of Rays," R. Naido, Annales De Physiquie 11, 72 (1934).
Known types of radioactive materials used in prior art ionization-type sensors or chambers have radiated at relatively high energy levels. For example, Americium241, Am 241, a known radioactive isotope used in prior ionization chambers radiates at 5.5 MeV. This material as is also known emits some percentage of gamma radiation along with the preferred alpha particles. The alpha particles are the useful particles in connection with detection of smoke.
The known Bragg curve, FIG. 1 hereof, makes it clear that the most productive portion, in terms of generation of ion pairs, of an alpha particle emitted from an uncoated Am241 sample takes place on the order of 3.5 centimeters from the source. This in turn dictates that an ionization-type sensing chamber which locates the radiation material at the sensing electrode should extend at least on the order of 3.5 centimeters from the sensing electrode to the electrode associated with the active region. Similar comments apply to the height of the reference chamber.
If the source is coated, the distance may be less than 3.5 cm. While coating the source may reduce this distance, known 2 micron coatings tend to reduce radiation in a conically shaped volume adjacent to the surface of the source. This represents a significant loss of potential operating current and as a result is undesirable.
The required 3.5 centimeters of travel for optimal generation of ion pairs, as illustrated in FIG. 1, for each of the chambers leads to the conclusion that it may be very difficult to reduce the overall height of a smoke detector, which incorporates an uncoated Am241 source, as much as desired.
It would be desirable if ionization sensors could be configured to have an overall height less than the height noted above without a significant loss in operating current. Preferably, such chambers could still be used with conventional smoke detector control circuitry and without a need for different types of manufacturing skills than theretofore has been required in the manufacture of ion chambers.
Another trend in the fire protection industry has centered upon minimization of the total activity contained in smoke detectors. Reduction of radioactivity inside ionization chambers also tends to reduce the operating current, which already has been reduced to a low level. Therefore, a significant opportunity for improvement of ionization chambers rests in the responding to a challenge for a shorter ionization chamber (on the order of 2 cm), with less radioactivity (roughly 5000 Bq), and with reliable levels of operating current (roughly 10 pA).